Recent years have seen progress in research and development of nonvolatile memory devices having memory cells each including a so-called variable resistance nonvolatile memory element (hereinafter, also simply referred to as a variable resistance element). Here, the variable resistance nonvolatile memory element refers to an element which has a characteristic of changing its resistance value according to an electrical signal and which can further store data corresponding to the resistance value in a nonvolatile manner. More specifically, the variable resistance nonvolatile memory element is a nonvolatile memory element whose resistance state reversibly changes between a high resistance state and a low resistance state according to a polarity of an applied voltage.
A nonvolatile memory device which is formed by arranging, in a matrix, so-called 1T1R (1 transistor 1 resistor) memory cells in each of which a MOS transistor and a variable resistance element are connected in series at a crosspoint between corresponding ones of bit lines and word lines that are placed orthogonally to each other is generally known as the nonvolatile memory device including such variable resistance elements (see Patent Literature 1, for instance).
Patent Literature 1 discloses a nonvolatile memory device including 1T1R memory cells in each of which a variable resistance element is made of an oxide having a perovskite crystalline structure.
Furthermore, in addition to a 1T1R (1 transistor 1 resistor) memory cell array, a memory cell array having a so-called crosspoint structure is generally known. In the crosspoint structure, each of the memory cells is provided at a crosspoint between corresponding ones of bit lines and word lines that are placed orthogonally to each other, so as to be placed between the corresponding ones of the bit lines and the word lines.
Patent Literature 3 discloses a nonvolatile memory device in which bidirectional variable resistance elements are used as memory cells. Patent Literature 3 discloses that a varistor, for example, is used as a bidirectional nonlinear element for a diode of a memory cell, so as to reduce a so-called leakage current flowing in a non-selected cell. Patent Literature 3 also discloses the crosspoint structure.
Patent Literature 2 discloses a nonvolatile memory device having a memory cell array including variable resistance elements having a multilayer structure and a three-dimensional crosspoint structure.
Non Patent Literature 1 discloses a memory cell structure in which a variable resistance element layer and a unidirectional diode are combined. Non Patent Literature 1 also discloses the multilayer structure.
In the meantime, there is a problem that when an excessive voltage is applied to or an excessive current is passed through a nonvolatile memory element having a variable resistance element layer, a resistance value of the nonvolatile memory element changes so significantly that the nonvolatile memory element does not show a resistance change.
In view of such a problem, it is intended to achieve a more stable operation by limiting the voltage or the current (see Patent Literature 4, for example). Patent Literature 4 discloses that a parallel resistance circuit or a series resistance circuit is provided to the outside of a memory cell, so as to prevent an excessive voltage from being applied to and an excessive current from being passed through the memory cell.